# Expanded scale Voltmeter

#### Frodofrog27

Joined Jan 16, 2006
1
There seems to be a number of ways of achieving this solution.

Up until now I have used a zener in series with the meter and calibration resistor, with a second resistor in parallel beforehand to control the zener firing voltage and a third high wattage resistor in either suply leg to limit the power.

I now wonder if this is far too complex, because it takes ages to balance the circuit. It also ends up with poor linearity.

Maybe a straight forward zener would do, with the calibration resistor. Use a 1mA movement and back the bottom bridge off so it operated from 0.7 to 1.7mA. That should make sure the zener fires okay.

Has anyone got any comments or suggestions on a better solution?

many thanks

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by Frodofrog27@Jan 16 2006, 04:20 AM
There seems to be a number of ways of achieving this solution.

Up until now I have used a zener in series with the meter and calibration resistor, with a second resistor in parallel beforehand to control the zener firing voltage and a third high wattage resistor in either suply leg to limit the power.

I now wonder if this is far too complex, because it takes ages to balance the circuit. It also ends up with poor linearity.

Maybe a straight forward zener would do, with the calibration resistor. Use a 1mA movement and back the bottom bridge off so it operated from 0.7 to 1.7mA. That should make sure the zener fires okay.

Has anyone got any comments or suggestions on a better solution?

many thanks
[post=13169]Quoted post[/post]​
If you can post a schematic of your present circuit and provide a description of your application, there is a good chance that one of our members can suggest a few possible ways to improve on what you have designed so far.

hgmjr

#### n9xv

Joined Jan 18, 2005
329
Is this an analog meter or DMM. If its an analog meter then you need a multiplier resistance between the voltage your measuring and the meter. The calculation is really very simple.

Rmult = Vfsd / Ifsd

Rmult is multiplier resistance.
Vfsd is full scale deflection voltage.
Ifsd is full scale deflection current.

Vfsd is the level of voltage that causes the meter to deflect full scale when connected directly to the voltage source. Its the full scale reading you "want" the meter to indicate.

Ifsd is the nominal value of current indicated on the meter face when deflected full scale.

Example:

Lets say the meter itself is a 50-uA meter and the voltage you want it to indicate at full scale is 100-volts. 100-volts / 50-uA = 2-megohm multiplier resistance.

#### pebe

Joined Oct 11, 2004
626
It depends what you mean by 'Expanded scale Voltmeter'.
Do you mean that you want to increase the range, ie. change 0-10v to 0-50v?
Or expand a section of the range, ie. change 0-10v to 8-10v?